I like Scott's advice...if it works for you....and there are "boat" load of lines on the market. Reviewing the offerings by Scientific Anglers, Rio, and Cortland (the 3 manufacturers in this country) can be a little mind-boggling. I'd recommend staying with one of those brands. Then, if you get into studying the profiles of the lines and the specs on the taper and body demensions of both WF and DT lines, it can become even more of a challenge to make a decision.
Some of what you wind up using is based on the inventory of shops and stores in your areas. Some of the chain stores or general tackle shops will carry the lower priced, more economical lines. Specialty fly shops probably tend to gravitate toward the premium lines in the $65+ range. Typical advice from fly shops and literature is to buy a premium line...since it and the rod are the key ingredients in performance (along with your skill...) The premium lines should be offering the latest in platicizers, UV resistance, floatability, slickness and the latest advances in the development of variations on standard WF and DT tapers and color changes and high-floating tip materials. Whew!
Some of the higher priced lines don't alway work out. (after it was introduced several years ago, Cortland 555 was a hit in some parts of the country and a disaster in other areas...Rio Gold has been "improved" for this year) Generally, however, I can usually always recommend a more expensive line with confidence that it will perform to a customer's satisfaction, especially if we consider when and how the line is going to be used most of the time. Will it be a line for wide open water like rivers, ponds and sounds...or is it going to be a line for short cast on wild trout streams? Is the customer begining fly fishing...or a veteran with good casting technique... And, if it doesn't work out okay, I'll swap it out or in some cases the company has given the customer a new line.
My best selling lines are the SA GPX (and lately their textured series), the Rio Grands and Rio Golds, and the Royal Wulff Triangle Tapers (which are usually my pick) . The Rio Mainstream lines are at the $39 price point and they have also done quite well, especially since they offer some of the premium tapers designs in a moderately priced line, both for trout and bass...especially for bass etc. I've carried an imported line by Flycast Outfitters, which has done well for about $22, and continue to stock the SA Air Cel for that price point and to help meet a customer's budget. I also continue to stock the longtime favorite, Cortland 444 in peach color...since a lot of die-hard fly fishermen refuse to use anything else.
Will a premium line give you 10 more feet of distance?...sometimes yes...sometimes no. But a clean line, improved technique and "hauling" (and maybe a different rod) certainly will. And, just for the sake of argument ( I meant to say discussion) ...a traditional WF and DT line will both perform about the same out to distances of about 35 feet..and if you are trying to roll cast that distance or more, the DT will excel. If economy is a primary objective, the DT line can be reversed when the front half is worn out. Having said that, WF lines are still the big seller...